1. Technical Field of the Invention
The present invention is directed to improvements for distributed antenna systems and more particularly to methods and systems for improving uplink communications.
2. Description of the Prior Art
Distributed Antenna Systems (DAS) are used to provide and/or enhance coverage for wireless services such as Cellular Telephony and Medical Telemetry inside buildings and campuses. The general architecture of a DAS is depicted in FIG. 1.
The DAS 100 typically includes a source 102 (a transmitter, receiver or transceiver for sending and/or receiving a signal), a main aggregation point 104, one or more remote or intermediate aggregation points 122, 124, 126 and two or more antennae 132, 134, 136 connected to the remote/intermediate aggregation points. The DAS can also include one or more terminals 140 for transmitting signals to and receiving signals from one or more of the antennae.
In referring to the signal flows in DAS systems, the term Downlink signal refers to the signal being transmitted by the source transmitter (e.g. cellular base station) to the terminals and the term Uplink signal refers to the signals being transmitted by the terminals to the source receiver.
Most wireless services have both an uplink and a downlink, but some have only a downlink (e.g. a mobile video broadcast service) or only an uplink (e.g. certain types of medical telemetry).
One measure of signal quality for a wireless signal is its Signal-to-Noise-Ratio (SNR). It represents the ratio of the relative power level of the desired signal to the power level of the undesired noise in the bandwidth specific to that signal. The higher the SNR, the “better” the signal is. Every receiver requires a minimal level of SNR in order to be able to correctly demodulate and/or decode the received signal. In the vicinity of the transmitter (such as a wireless terminal) which emits the signal, the SNR would typically be very high.
On its path to the receiver, the SNR of the uplink signal can decrease in one of two ways. In one way, the level of the signal is attenuated while the noise level remains constant. This is typical of the propagation of the signal in the air and through any passive elements of a DAS system. In another way, the level of the signal remains constant or is increased, but the noise level increases even more. This is typical of the propagation of a signal through active elements in a DAS such as amplifiers.
It should be noted that when multiple uplink paths are combined, as is the case in virtually every DAS, the resulting noise level is a combination of the noise levels of the different paths, while the resulting signal level would typically be equal to the highest signal level encountered on any single path (the typical situation would be for the signal level to be high on one branch of the DAS, corresponding to the area where the terminal generating the signal is located, and very low or non-existent on other branches). As a result, aggregating multiple uplink paths increases the noise level and therefore reduces the SNR of the uplink signal.
A qualitative analysis of the degradation of the uplink SNR can be achieved by tracing the path of the uplink signal from terminal to receiver. A generic diagram of an uplink path 200 is depicted in FIG. 2. Each such stage of the signal path can have an impact on the SNR.
Propagation through the air 212 attenuates the level of the signal, and the attenuation is greater for higher frequency signals than it is for lower frequency signals. The noise level, in the best case, is the inherent environmental noise, usually referred to as Thermal Noise. Typically, the level of this noise is −174 dBm/Hz. In some cases, the environment may exhibit a higher level of noise in the specific frequency range in which the signal of interest operates.
Generally, most of the SNR deterioration occurs in this segment of the path, since the signal attenuation in the air 212 is typically much worse than in any passive segment of the DAS (e.g. coax cable). One way to decrease the signal attenuation in this segment is to shorten the maximal distance between a terminal and the closest DAS antenna to the terminal, or increase the number of antennas covering each area (and thus increase their density and decrease the distance between them). However, there is a cost penalty associated with doing this.
The passive section 214 of the DAS 200 is defined as the series of passive elements (antenna, cables, filters, combiners, attenuators, etc.) that precede the first active gain element (amplifier) in the uplink path. Propagation through the passive section 214 of the DAS 200 typically attenuates the level of the signal. However, the antenna may increase the signal level, if it has positive gain in the frequency range of interest. In a well designed DAS 200, the passive section 214 will maintain the level of noise at the Thermal level and might possibly reduce the noise to the Thermal level if, for example, it was higher coming into this section. On balance, the passive section 214 will typically decrease the SNR of the signal, more severely impacting signals of higher frequency.
Within the active portion 126 of the DAS 200, the level of the signal can be controlled by adjusting the gain of the amplifiers in the different system elements. An amplifier however, introduces a minimal noise level which is typically higher than the Thermal noise level and increases commensurate with the gain of the amplifier, and thereby will always decrease the SNR of the signal.
At each aggregation point 218, the noise levels of the different branches are combined. Since typically each uplink signal originating from a specific terminal will be present on only one of the DAS 200 branches being combined in the aggregation point 218, the signal level of the combined signal (assuming unity gain) will be the same. The signal level does not change, while the noise level increases, and therefore the SNR is impaired. In an aggregation point 218 with N branches the increase in noise level, and therefore the decrease in SNR, expressed in dB would be 10 log N.